1. Field of the Invention
The invention relates to the field of virtual local area networks (VLANs). In particular, the invention relates to inter-domain loop protection for VLANs with multiple spanning tree protocol domains.
2. Background Information and Description of Related Art
A virtual local area network (VLAN) restricts communication between computers in a local area network (LAN) on some other basis than physical location of the computers. The computers in the VLAN communicate with each other as if they are connected to the same LAN segment even though they may actually be physically located on different segments of a LAN.
LANs are often partitioned into separate segments to increase network performance. Bridges or switches connect the segments. A LAN may include redundant bridges and paths to provide backup in the event of a bridge or path failure in the LAN. The Spanning Tree Protocol (STP), part of the IEEE 802.1D standard, comprises an algorithm that provides for this redundancy while preventing undesirable loops in the LAN. Using the algorithm, bridges select a root bridge, and calculate and select a single path to the root bridge when faced with multiple redundant paths. After path costs are calculated and a single path is selected, the bridges block the redundant links.
The STP algorithm defined in IEEE 802.1D has been modified in different ways to further accommodate VLANs. For example, the IEEE 802.1Q standard specifies a single STP instance for all VLANs while another approach provides for a separate STP instance for each VLAN. Yet another approach contemplates one or more STP instances for one or more VLANs. Thus, an STP instance or STP domain (STPD) may span multiple VLANs, or a single VLAN may encompass multiple STPDs. However, one problem with multiple STPDs is the possibility of inter-domain loops through the switches that are members of more than one STPD, since the STP algorithm only guarantees a loop-free topology inside each domain.